A high-capacity backbone communication line is demanded as a communication speed of a communication line such as the Internet increases. Wavelength division multiplex (WDM) techniques are used in the backbone communication line.
A relay apparatus of a communication network and a transfer apparatus such as a terminal apparatus extracts a clock signal from an input signal, and synchronizes a signal to the extracted clock signal. An oscillator is included in the transfer apparatus. The oscillator internally generates a clock signal if the input signal is interrupted.
A signal transfer apparatus is conventionally known which includes a plurality of programmable oscillators and outputs a signal of any frequency by summing output signals of the programmable oscillators (Japanese Laid-open Patent Publication No. 63-061594).
In another known technique, a correction Read Only Memory (ROM) storing correction digital data correcting an oscillation frequency is arranged in a programmable oscillator, and variations in the resistance value of a resistor and the capacitance value of a capacitor in the programmable oscillator are corrected by the correction ROM (Japanese Examined Utility Model No. 2527832).